The construction of the shield and the hydraulic jacks used for its advance are explained in a [preceding chapter].
In very loose soils, a solid bulkhead of masonry is built across the tunnel, after the shield has advanced to a certain distance and some rings of the cast-iron lining have been erected. The bulkhead is provided with three air locks—two near the floor of the tunnel, for working purposes, and one near the roof, called the emergency lock, which, as the name suggests, is used only in case of danger. The air locks are steel cylinders from 10 to 15 ft. long and 6 ft. in diameter, made up of boiler plates. They are provided with doors at each end, besides the pipes for the admission and exit of compressed air. The working locks also have narrow-gauge tracks for hauling purposes. In rock or more consistent soil the bulkhead is constructed after the shield is far ahead, since there is no immediate necessity, under these conditions, to use the compressed air. In both the loose and good soils, when the shield has been advanced over 500 ft. from the bulkhead, a second bulkhead, with air locks, is erected in the tunnel. The first is left in place but used only in case of emergency.
To direct the shield along the center line and through curves and grades, accurate measurements are taken, and the distance between the shield and the last ring inserted in the iron lining is regulated accordingly. The alignment inside the tunnel is maintained in a very simple way. For this purpose, points corresponding to the center line are marked on the roof at distances of 100 ft. Nearly 100 ft. from the shield, a transit is set up on a strong scaffold spanning the tunnel, and it is supported by the flanges of the iron lining. A plumb-line is hung from one of the points of the roof already determined, as indicating the center line; and the transit man aligns his instrument with this plumb-line; after this he “plunges” his telescope. A rodman next places a horizontal rod of special construction between the flanges of the last ring of the lining. This rod has in the center an open slot which carries a glass with a black vertical line. The slot is graduated, the zero of graduation remains in the center while the vertical line is moved right and left. The rodman places a lamp behind the slot and the transit-man tells him how to move the dark line until it coincides with the axis of the tunnel. If the ring, just erected, be a little out of alignment, it is readjusted by pushing the shield a little more on the side that has swerved from the axis of the tunnel. As the shield is pushed forward, it is kept in place by four men with graduated rods, one man on each side of the shield, one on top and the other on the floor. As the shield progresses, they repeat aloud in succession, the distance indicated on the rods, which is the distance from the shield to the outer circumferential flange of the last ring of the lining. When an advance of one foot has been made, readings are taken at every inch; and when very near the required distance, they are taken at every quarter of an inch. In this way it is not difficult to bring the shield back into line, in case it may have shifted a little to the right or left. When curves are met, the rings are no longer cylindrical segments but tores, so that the segments at one side are longer than those on the other. In this case, the shield is advanced more on one side by a quantity equal to the difference of the two sides of the ring to be erected. At each advance the shield is moved 2 ft. or 21⁄2 ft. ahead, the distance corresponding to the length of the cast-iron rings of the lining. Within the space now open between the shield and the lining another ring is inserted. The ring is composed of different segments provided with flanges and holes bored so they can be bolted together. The segments of the lining are very heavy and difficult to handle but they are easily set by means of the erector.
When the erector is not mounted on the shield, it is located in the middle of a girder placed across the iron rings of the lining and just at the rear end of the shield. The girder, at both extremities, has flanged wheels resting on rails which are placed on brackets. These brackets are attached temporarily to the flanges of the iron lining. The erector is provided with an arm capable to swing in a full circle. Its movements are regulated by two hydraulic jacks, located horizontally on the spanning girder. On the extreme end of the revolving arm are projections with holes for the bolts. Each segmental plate of the lining has a kind of plug in the center which is cast together with the plate and is provided with holes for the bolt. In placing the segmental plates of the lining, the arm of the erector is swung over the plate to be lifted, then two bolts are passed through the holes in the projection of the erector, and through those in the plug. The arm of the erector is then moved upwards until the plate, free from all obstacles, is swung very near its intended position. There it is adjusted and held until bolts are inserted to fix it to the plates of the preceding ring.
In connection with the method of excavating submarine tunnels by means of shield and compressed air, the excavation varies with the quality of soil encountered. In compact rock the usual heading and bench method, so common in land tunnels, is also employed in this case. The shield is left behind in presence of good rock.
The men at the front attack the rock with air drilling machines and charges of dynamite. The holes are driven at a smaller depth than in land work; very light charges of dynamite are used and only a few holes fired at each round. Every precaution is taken in order not to disturb the shield and the bed of the river any more than is possible, because at a shallow depth the blast would tend to widen the existing crevices in the rock and thus permit an inflow of water. When the rock is fissured or disintegrated and the roof of the excavation at the front requires timbering, the shield should be kept closer to the front. In this way the quantity of timber for strutting is greatly reduced, so lessening the probabilities of fires. It is very difficult, in compressed air, to extinguish fires and in almost every instance the only way is to flood the tunnel. This was done at the Manhattan end of the tunnel under the East River for the extension to Brooklyn of the New York Subway.
The excavation is made by hand in loose but compact soils such as clay. The men work on platforms located at the front of the shield and they are protected from the caving-in of the roof by a hood added for working through loose soils. The men excavate the material which is shoveled inside the tunnel and is carried away in small cars. The shield is very close to the front of the excavation in loose soil. The East Boston tunnel, under Boston Harbor, connecting with the Boston Subway, was excavated through blue clay. The minimum distance between the bottom of the water and the roof of the excavation was 18 ft. The tunnel was excavated by means of compressed air and the shield which was only used for the roof. It slid on top of concrete side walls built in two drifts which were excavated nearly 100 ft. ahead of the shield. The tunnel was lined with concrete, the arch being reinforced by longitudinal steel rods which received the thrust of jacks used for advancing the shield. The material in the drifts under the shield and the bench was removed by hand and carried away in small cars.
Subaqueous tunnels driven through very loose soils can be excavated by simply leaving the doors open while the shield is pushed ahead. The material, dislodged by the cutting edge of the shield, is forced through the doors and falls on the floor whence it is removed in small cars. In very loose soils the excavation has been made in a still more economic way; the shield with closed doors is simply squeezed through the soil. This method is financially convenient, because all the excavating and hauling operations are eliminated and the tunnel progresses from 40 to 50 ft. per day, but clearly indicates a lack of stability. In this manner, the Hudson River tunnel of the New York and New Jersey Railroad was constructed.
The pressure of the air in the tunnel depends upon the depth and as a rule it varies between 20 and 40 or even more pounds per square inch above atmospheric pressure. Working in compressed air causes a peculiar disease commonly known as “bends” or “caisson disease” often proving fatal. To prevent and remedy the disease, the engineers should order a set of rules to be strictly observed. The preventative measures should be, first, to employ only sober, strong and healthy men, never one who has not successfully passed the examination of the attending physician; second, to order the lock tenders never to allow any man in or out of the tunnel unless he has spent at least ten minutes within the locks. Both compression and decompression should be thorough and it cannot be in less than this time. A stop of only a few minutes in the locks is not sufficient and this incomplete compression or decompression is the real cause of the bends. The men become careless after they have been in the compressed air for some time, and they try to reduce this tiresome operation to a minimum, hence the duty of the engineer to strictly enforce this rule. The remedial measures should consist of constant medical attendance near the shafts and the erection of a compressed air hospital where the men affected by bends for lack of decompression may be attended and cured.